1. Field of the Invention
The invention relates to a fuel cell system and an electric vehicle. More specifically, the invention relates to a fuel cell system including a fuel cell and an electric power storing means, and an electric vehicle including the fuel cell system.
2. Description of the Related Art
Improvement of energy efficiency is required in an electric vehicle including a fuel cell system. For example, Japanese Patent Laid-Open Publication No. 2001-307758 discloses a fuel cell system in which a required amount of electric power is supplied from a fuel cell and a secondary battery when a load is higher than a reference value, and the required amount of electric power is supplied from the secondary battery with operation of the fuel cell being stopped when the load is equal to or lower than the reference value. In the fuel cell system, the fuel cell is operated intermittently such that operation of the fuel cell is temporarily stopped in a low load region where efficiency of the fuel cell system decreases, and the fuel cell is operated in a region where energy conversion efficiency is high, whereby efficiency of the entire fuel cell system is improved.
Although the efficiency of the entire fuel cell system is improved by intermittently operating the fuel cell, a load fluctuates frequently in an electric vehicle. Therefore, in the case where the fuel cell is intermittently operated, when operation of the fuel cell is temporarily stopped and electric power supply from the fuel cell is temporarily stopped, the operation of the fuel cell needs to be restarted in a short time. Accordingly, an air compressor and a hydrogen pump (auxiliary machinery) may be operated so that open circuit voltage (OCV) at an output end of the fuel cell can be maintained at a predetermined value even while electric power supply from the fuel cell is stopped. In this case, however, hydrogen (fuel) is consumed by the fuel cell in order to maintain the open circuit voltage, and accordingly fuel efficiency deteriorates.